Proton transfer and polarizability of hydrogen bonds formed between cysteine and lysine residues

Abstract

(L‐Cys)n + N‐base systems and (L‐Cys)n + (L‐Lys)n systems were studied by ir spectroscopy. It is shown that in the water‐free systems, SH ⃛N ⇌ S− ⃛H+N hydrogen bonds are formed. With the (L‐Cys)n + N‐base systems, both proton‐limiting structures in the SH ⃛N ⇌ S− ⃛H+N bonds have equal weight when the pKa of the protonated N‐base is 2 pKa units larger than that of (L‐Cys)n. The same is true with the water‐free (L‐Cys)n + (L‐Lys)n system. Thus, with regard to the type of proton potentials present, these hydrogen bonds are proton‐transfer hydrogen bonds showing very large proton polarizabilities. This is confirmed by the occurrence of continua in the ir spectra. Small amounts of water open these hydrogen bonds and increase the transfer of the proton to (L‐Lys)n. In the (L‐Lys)n + N‐base systems, with increasing proton transfer the backbone of (L‐Cys)n changes from antiparallel β‐structure to coil. In (L‐Cys)n + (L‐Lys)n, the conformation is determined by the (L‐Lys)n conformation and changes depending on the chain length of (L‐Lys)n. Finally, the reactivity increase in the active center of fatty acid synthetase, which should be caused by the shift of a proton, is discussed on the basis of the great proton polarizability of the cysteine–lysine hydrogen bonds.

Preferred Hydrogen-Bonding Partners of Cysteine: Implications for Regulating Cys Functions

“…They predict that, in a solvent-inaccessible environment, a Cys 0 ···Lys 0 HB might be more stable than a Cys – ···Lys + HB, whereas the latter is more stable than the former in an aqueous medium. Interestingly, this prediction is supported by IR spectroscopy, which shows that, in the absence of water, both Cys 0 ···Lys 0 and Cys – ···Lys + structures were found, but hydration tipped the equilibrium in favor of the Cys – ···Lys + salt-bridged structure.…”